Method of anodizing aluminum with electrolyte containing sulfophthalic acid



United States Patent METHSD OF ANODEZTNG ALUMINUM WIIH ELECTROLYTE CGNTAINING SULFOPHTHALIC ACID William P. Kampert, Springdale, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 24, 1961, Ser. No. 147,177

Claims. (Cl. 20458) This invention relates to anodizing aluminum, and particularly to producing oxide coatings on an aluminum article by making the same anode in an electrolytic cell employing an aqueous acid electrolyte. As used herein, the word aluminum includes high purity aluminum, aluminum in various commercial grades, and aluminum base alloys.

Decorative and protective oxide coatings on aluminum have long been made by anodizing in electrolytes consisting of aqueous solutions of sulfuric acid. Such electrolytes are usually employed at a temperature near room temperature, and must be cooled to maintain such temperature. The coatings, as so produced, are usually relatively clear or colorless, although on some alloys they may be tinted or colored by some constituents.

In recent years, hard anodic oxide coatings which are more abrasion resistant, have been produced by anodizing in aqueous sulfuric acid electrolytes at extremely low temperatures, e.g. 25 to 32 F., requiring much more cooling, with consequent increased equipment and operating expense. Such coatings, as so produced, in addition to being abrasion resistant, are colored in metallic gray, brownish gray and dark brown colors, which are attractive for architectural and other uses of aluminum. Consequently, more practical methods of producing similar coatings are demanded.

It is a general object of this invention to provide, for use in an electrolytic cell for anodizing aluminum, 8. novel electrolyte and anodizing process for producing oxide coatings on aluminum, particularly colored oxide coatings. Another object is to provide such an electrolyte and process suitable for use under a wide range of practical operating conditions. A particular object is to provide such an electrolyte and process for anodizing aluminum with moderate cooling of the electrolyte. Another object is to produce abrasion resistant, colored anodic oxide coatings on aluminum at moderate cost. A specific object of the invention is .to produce abrasion-resistant, colored anodic oxide coatings on aluminum, with moderate voltage and cooling requirements.

In accordance with the invention, it has been found that an electrolyte consisting essentially of at least one sult'ophthalic acid selected from the group consisting of 4-sulfophthalic acid and S-sulfoisophthalic acid, together with sulfate provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water, may be advantageously used in an electrolytic cell for anodizing aluminum.

The 4-sulfophthalic acid, or the S-sulfoisophthalic acid, or a mixture thereof, may be employed in a very small amount, e.g. less than 10 grams per liter, but prefer-ably at least 10 grams per liter or more are employed. The solution limit lies near 55 percent by weight, in the case of 4-sulfophthalic acid, for example, and even a substantial undissolved excess does not prevent anodizing. For production of colored coatings at or near room temperature, at least 50 grams per liter is desirable, and very satisfactory results have been obtained when 4-sulf0- Patented Jan. 4, 1966 pht-halic acid or 5-sulfoisophthalic acid is present in an amount of about 70 to 130 grams per liter. These sulfophthalic acids are more expensive than sulfuric acid, so larger amounts would not orinarily be preferred from a cost standpoint.

The sulfuric acid, or equivalent sulfate, or mixtures thereof, may likewise be employed in a very small amount, e.g. less than 1 gram per liter (calculated as H may be employed, but preferably at least 1 gram per liter or more is employed. Somewhat larger amounts will ordinarily be employed, since the voltage requirements for the same anodizing current density (with consequent ele-ctrical equipment, and electrolyte cooling cost factors to be considered) are reduced in such cases. However, for the production of colored coatings at or near room temperature, an amount less than 20 grams per liter is desirable, and very satisfactory results have been obtained when sulfuric acid or soluble sulfate is present in an amount of about 3 to 10 grams per liter. The sulfate content may be conveniently provided by sulfuric acid, an alkali metal or ammonium sulfate or bisulfate such as sodium sulfate, sodium bi-sulfate or ammonium bisulfate, a heavy metal sulfate or bisulfate such as ferrous sulfate, an organic sulfate or bisulfate such as aniline sulfate, or any other water soluble sulfate or bisulfate such as hydrazine sulfate. Double sulfates may also be used.

While various proportions of the sulfophthalic acid and the sulfate may be used, it has been found that the production of uniform, colored coatings at or near room temperature is most readily obtained when the ratio of sulfophthalic acid to sulfate (calculated as H 80 is at least about 7:1. Electrolytes with lower ratios of these acids even less than 1:1, are also useful, however. Generally, with the lower ratios, lightly colored or almost colorless coatings may be obtained at or near room temperature. More abrasion resistant, colored coatings may be obtained at lower temperatures which are not, nevertheless, required to be as low as with sulfuric acid electrolytes devoid of the sulfophthalic acid.

The following is a specific example of a preferred electrolyte suitable for use in accordance with the invention:

Electrolyte A Component: Grams per liter 4-sulfop'hthalic acid Sulphuric acid 8 Water Balance Grams per Liter Component B C D 4-sulfophthalic acid 0 10 75 5-sulfoisophthalic acid 88 0 0 Sulfuric acid 7 1 Water Balance in each case Other specific electrolytes suitable for producing a lightly colored coating at room temperature or darker colored Grams per Liter Component -sulfophthalic acid Sulfuric acid 100 Balance in each case As indicated above, electrolytes for use in accordance with the invention may beemployed ,under a wide variety of operating conditions. A temperature at or near room temperature, e.g. 75 B, may be employed. However, either extremely low temperatures or rather high temperatures may also be employed, i.e. temperatures between about 40 and 110 F. Very satisfactory results in producing colored coatings with preferred electrolyte compositions such as Electrolytes A, B or D, abovementioned, with a moderate amount of cooling being required, have been obtained with temperatures between about 60 and 90 F.

Current densities may extend over a wide range, for practical purposes as low as-9 amperes per square foot or even less, and as high as 144 amperes per square foot or even more, depending on size, shape and composition of the aluminum article. Preferably,,the current density is between 12 and 36amperes per square foot. The current may be either AC. or DC, or a current of undulating characteristics, but D.C. is quite satisfactory.

Time of treatment depends upon current density and thickness of coating desired, as coating thickness is generally a function of anodizing current density and time. Abrasion resistant coatings for outdoor service are often made in a thickness of- 0.4 to 1 mil (0.0004 to 0.001 inches). However, coatings may be made in any substantial thickness desired, e.g. 0.1 mil or greater.

Illustrative of particularly suitable operating conditions, Electrolyte A above mentioned has been operated at 75 F., at 24 amperes per square foot, with voltages starting at about 32 volts and generally extending upto about 70 volts over a 30 minute period, depending on the composition of the aluminum being coated, to produce abrasion resistant, colored oxide coatings on a variety of aluminum articles. As examples of the colored coatings produced on various types of aluminum, it is notedthat Electrolyte A, operated under the conditions just noted, for time suflicient to produce coatings about 1 mil thick, produced the following results as to color:

Color stability of the colored coatings was very high when tested in the Fade-O-Meter after 1000 hours of exposure. Lighter and darker shades of these colors were obtained with thinner and thicker coatings, respectively. Similar results were obtained on 6061-T6 and 6063-T alloys with Electroyte B and the same operating conditions.

As an example of the abrasion resistance of such coatings, produced on 1100-H14 aluminum under the same conditions except as to choice of current density which was reduced to a less favorable level of 12 amperes per square foot, an abrasion resistance value of 400 grams per mill was obtained by the ASTM D658-44 method of test.

Coatings produced in accordance with the invention may be sealed, or dyed or pigmented and sealed, by conventional procedures. Thus, dyes or pigments may be used to modify the colors produced by the anodizing methods described herein. Etching or brightening treatments may be given the aluminum surfaces prior to anodizing, when desired.

What is claimed is:

1. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of at least 10 grams per liter of at least one sulfophthalic acid selected from the group consisting of 4-sulfophthalic acid and 5-sulfoisophthalic acid, at least 1 gram per liter and less than 20 grams per liter of sulfate (calculated as H provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water, the ratio of sulfophthalic acid to sulfate (calculated as H 50 being at least 7:1,

for a time suflicient to produce a colored anodic oxide coating at least 0.1 mil thick.

2. A method in accordance with claim 1 in which the electrolyte is maintained at a temperature between about 40 and F., and

the current density is maintained between about 9 and 144 amperes per square foot for a time sufficient to .produce a colored anodic oxide coating at least 0.4

mil thick.

3. A method in accordance with claim 2 in which the sulfophthalic acid is 4-sulfophthalic acid in an amount of at least 50 grams per liter, and the sulfate is provided by sulfuric acid.

4. A method in accordance with claim 2 in which the sulfophthalic acid is 5-sulfoisophthalic acid in an amount of at least 50 grams per liter, and the sulfate is provided by sulfuric acid.

5. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of about 70 to grams per liter of 4-sulfophthalic acid, about 3 to 10 grams per liter of sulfate (calculated as H 50 provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water,

while the electrolyte is maintained at a temperature of between about 60 and 90 F., and

the current density is maintained between about 12 and 36-amperes per square foot,

for a time of treatment suflicient to produce an anodic oxide coating at least 0.4 mil thick,

whereby a colored anodic oxide coating is produced on the surface of the aluminum article.

6. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of about 70 to 130 grams per liter of 5-sulfois0phthalic acid, about 3 to 10 grams per liter of sulfate (calculated as H 50 provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water,

while the electrolyte is maintained at a temperature of between about 60 to 90 F., and

the current density is maintained between about 12 and 36 amperes per square foot,

for a time of treatment sufiicient to produce an anodic oxide coating at least 0.4 mil thick,

whereby a colored anodic oxide coating is produced on the surface of the aluminum article.

7. A method in accordance with claim 5 in which the sulfate is provided by sulfuric acid.

8. A method in accordance with claim 6 in which the sulfate is provided by sulfuric acid.

9. A method of anodizing aluminum, comprising making an aluminum article anode in an oxide coating productive electrolyte consisting essentially of at least 10 grams per liter of sulfophthalic acid, about 1 to 20 grams per liter of sulfate (calculated as H 80 and water, for a time sutficient to produce a colored anodic oxide coatmg.

10. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte con sisting essentially of at least 10 grams per liter of sulfophthalic acid consisting essentially of 4-sulfophthalic acid, at least 1 gram per liter and less than 20 grams per liter of sulfate (calculated as H 50 provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water,

References Cited by the Examiner UNITED STATES PATENTS 3/1941 Korpiun 20458 4/1962 Deal et al. 204-58 10 ALLEN B. CURTIS, Primary Examiner.

JOSEPH REBOLD, MURRAY A. TILLMAN,

Examiners. 

9. THE METHOD OF ANODIZING ALUMINUM COMPRISING MAKING AN ALUMINUM ARTICLE ANODE IN AN OXIDE COATING PRODUCTIVE ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST 10 GRAMS PER LITER OF SULFOPHTHALIC ACID, ABOUT 1 TO 20 GRAMS PER LITER OF SULFATE (CALCULATED AS H2SO4), AND WATER, FOR A TIME SUFFICIENT TO PRODUCE A COLORED ANODIC OXIDE COATING. 